Problem with a PMOS in single->dual power supply splitter

Hello!
I was assembling the circuit in the attachment’s schematics on single to dual power splitting.

I am using the exact MOSFET in the SUP90P06-09L-E3 diagram but I am experiencing a problem.

In practice, the voltage exiting the mos drain is equal to that of the source (V +), making rail splitting useless. In fact, I find myself again in a situation of + 20V / 0V instead of + 10/0 / -10.
Practically the GND and V + have the same potential … what am I doing wrong?
I specify that if I do not connect the MOS, that is, with only the voltage follower (and voltage divider) the voltages are divided correctly … so the problem I imagine derives from the MOS section …
Thank you all in advance :slight_smile:

single to dual.png

The LM358 is not a rail2rail opamp (max positive out is VCC -1.5volt).
It's output can't reach V+, so the mosfet will never turn fully off.

Maybe better to use a PNP/NPN emitter follower pair at the output of the opamp.
Or use the logic level fet in common source configuration (depending on supply loads).
Leo..

Couple thoughts,

  1. There is an inversion in your circuit. Just to make the analysis easier, for now lets call
    the output -V --> GND
    the output gnd --> V/2
    the output V+ stays V+

If V+ = 10V then V/2 = 5V

In operation if V/2 goes too low, the output of U2A goes high, turning the FET off and V/2 drops even lower.

  1. Once you fix the inverting issue, you have a opamp with a high gain ( voltage) FET at a closed loop of 1. You will likely have stability problems.

  2. I'll assume there is more to the circuit but if you are testing exactly what you've drawn then you need a load as your circuit can only drive in one direction.

Generating a virtual ground like that requires push-pull drive, not single ended like your circuit.

How much current is the dual rail circuit going to take from each of its rails to the virtual ground?

Hi,
What is Vin and is its negative connected to ground.
If so then your circuit will not work.
What is producing Vin?
single to dual.png
Tom.. :slight_smile:

Wawa:
The LM358 is not a rail2rail opamp (max positive out is VCC -1.5volt).
It's output can't reach V+, so the mosfet will never turn fully off.

Maybe better to use a PNP/NPN emitter follower pair at the output of the opamp.
Or use the logic level fet in common source configuration (depending on supply loads).
Leo..

Do you have some examples? Sorry I'm a bit noob in EE.

JohnRob:
Couple thoughts,

  1. There is an inversion in your circuit. Just to make the analysis easier, for now lets call
    the output -V --> GND
    the output gnd --> V/2
    the output V+ stays V+

If V+ = 10V then V/2 = 5V

In operation if V/2 goes too low, the output of U2A goes high, turning the FET off and V/2 drops even lower.

  1. Once you fix the inverting issue, you have a opamp with a high gain ( voltage) FET at a closed loop of 1. You will likely have stability problems.

  2. I'll assume there is more to the circuit but if you are testing exactly what you've drawn then you need a load as your circuit can only drive in one direction.

Hi John,
I didn't design the circuit by myself. I found it online on a website..I'm not that an expert in EE, I have some basic knowledge but nothing more.
However, with this circuit I plan to power supply an arduino(or other MCU Dev Boards) with a lot of sensors and displays, but i also need an opamp for proper audio amplification. I'm doing this as i need it far from home's 230VAC.

MarkT:
Generating a virtual ground like that requires push-pull drive, not single ended like your circuit.

How much current is the dual rail circuit going to take from each of its rails to the virtual ground?

I don't know exactly how much current but i guess like 750mA/1A...
With pushpull drive do you mean i need two MOSFETs?

TomGeorge:
Hi,
What is Vin and is its negative connected to ground.
If so then your circuit will not work.
What is producing Vin?
single to dual.png
Tom.. :slight_smile:

Vin is an external power source, in my case I have a "power bank" that supplies 20V/12V with 2A/3A. Yes the negative is grounded...

Hi,

Vin is an external power source, in my case I have a "power bank" that supplies 20V/12V with 2A/3A. Yes the negative is grounded...

If the neg of Vin is at gnd, then how can you get -V potential, using your circuit.
You have Vin neg/gnd connected to -V and gnd?

Just connect the positive terminal of your bank to +V
Connect negative terminal of your bank to -V, not gnd.
You bank needs to float, not be connected to gnd.

Tom... :slight_smile:

miki2o:
Do you have some examples? Sorry I'm a bit noob in EE.

Replace the mosfet with a push-pull complementary pair like this.
Bases to opamp output, emitters to virtual ground (opamp feedback).

If it will work depends on current draw and imballance of the bipolar supply.
What are you trying to do that requires boosting of the opamp current capabilities.
The opamp/power stage is only there for the differential current, not for the total current draw.

The classical way is two shunts in the power legs of the opamp, and the transistors driven from those voltdrops.
Fifth diagram from the bottom on this page.
Leo..

So now its time to ask.... what is the purpose of the negative voltage? Do you have an estimate of how much current you will need it to supply?

For clarity, consider your existing 20V supply has two terminals:

V+ or 20v+
and Common or 20V common.

the use of Negative for the common side of the 20V supply can be confusing when you are using it in a context where there will also be a negative potential (i.e. you V-)

John